Serialize Character values in XContent maps

[kalayciburak]

Description

XContentBuilder rejected Character values with cannot write xcontent for unknown value of type class java.lang.Character. Ingest scripts such as ctx.char = (char)'a' put a Character into the document map, and IngestService then failed while serializing that map.

Register Character in the WRITERS table and write it as a one-character string, matching Locale / Class.

Related Issues

Resolves #14382

Check List

• Functionality includes testing.
• API changes companion pull request created, if applicable.
• Public documentation issue/PR created, if applicable.

Test plan

Executed as a non-root user on JDK 21:

• RED: :server:test --tests org.opensearch.common.xcontent.builder.XContentBuilderTests.testWriteMapWithCharacterValue failed with IllegalArgumentException: cannot write xcontent for unknown value of type class java.lang.Character
• GREEN: the same test plus JsonXContentTests.testUnknownObject
• :server:test --tests org.opensearch.common.xcontent.builder.XContentBuilderTests --tests org.opensearch.common.xcontent.json.JsonXContentTests --tests org.opensearch.common.xcontent.smile.SmileXContentTests.testUnknownObject --tests org.opensearch.common.xcontent.cbor.CborXContentTests.testUnknownObject --tests org.opensearch.common.xcontent.yaml.YamlXContentTests.testUnknownObject
• spotlessJavaCheck

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[github-actions]

PR Reviewer Guide 🔍

Here are some key observations to aid the review process:

🧪 PR contains tests

🔒 No security concerns identified

✅ No TODO sections

🔀 No multiple PR themes

⚡ No major issues detected

[github-actions]

PR Code Suggestions ✨

Explore these optional code suggestions:

Category	Suggestion	Impact
General	Also handle primitive char array type Also register a writer for char[] so primitive character arrays are serialized consistently with other primitive array types (e.g., int[], short[]). Without this, a char[] value in a map would fall through to a generic handler or fail. libs/core/src/main/java/org/opensearch/core/xcontent/XContentBuilder.java [121] writers.put(Character.class, (b, v) -> b.value(v.toString())); +writers.put(char[].class, (b, v) -> b.value(new String((char[]) v))); Suggestion importance[1-10]: 5 __ Why: Adding a char[] writer for consistency with other primitive array types is reasonable, though the PR's scope is narrowly focused on Character handling. The suggestion is a valid enhancement but not critical, and serialization as a String may not be the desired behavior (vs. an array of chars).	Low

[github-actions]

❌ Gradle check result for 5975d68: FAILURE

Please examine the workflow log, locate, and copy-paste the failure(s) below, then iterate to green. Is the failure a flaky test unrelated to your change?

[iprithv]

LGTM